1. FIELD OF THE INVENTION
The present invention is directed to a subterranean well casing float tool, which may be either a float collar or float shoe, for use in a subterranean well for floating of casing and cementing operations.
2. BRIEF DESCRIPTION OF THE PRIOR ART
Subsequent to the drilling of a subterranean oil or gas well, casing is run into the well and secured to the borehole by a cementing operation which comprises introducing under pressure through the interior of the casing conduit or string a cementitious material which is pumped through the casing and out through its lower end, thence through an annular area defined by the borehole or wall and the outer diameter of the casing to stabilize the borehole, anchor the casing into the well and to isolate various geological formations or zones from one another within the well to prevent the production of water or other fluids with the desired hydrocarbons in the particular production or pay zone. In the running of the casing string into the well and during the cementing operation, there is need to provide a valving system within a collar positioned along the length of the casing or within a shoe, positioned at the lowermost end of the casing string.
During the lowering of the casing into the well, a float shoe is frequently positioned on the bottom of the casing conduit which is being lowered into the wellbore. The shoe has an end portion which is rounded and heretofore has been filled with cementitious material having a longitudinal passage therethrough within which there is positioned valving means for selectively opening and closing a flow path through the cementitious material and the valve structure as well as the casing conduit. The valve structure closes in an upward direction and opens in a downward direction. Accordingly, when the casing conduit is being lowered into the subterranean well, the valving structure is closed and the conduit is kept free of fluid, thus providing the buoyancy for flotation of the casing into the position for the cementing operation.
When the casing has been lowered into position whereby the lowermost end of the string is adjacent the lowermost end of the zone to be produced, or the like, the cement which is used for cementing the wellbore can be introduced through the casing for passage downwardly through the valving structure and out around the exterior of the lowermost end of the casing, thence upwardly to fill the wellbore around the exterior of the casing.
Float collars are sometimes utilized, either alone or in combination with the float shoe, for providing additional buoyancy. A float collar differs from a float shoe in that it has threaded connections on both ends for connection on one side to a casing conduit and on the other side to another collar, sub, or one end of a casing conduit member.
Typical of such prior art patents disclosing various valving structures for flotation and cementing operations include U.S. Pat. No. 1,776,613, to Baker, which discloses a well shoe for floating a casing into place. The float shoe which is shown has a downwardly opening check valve which is cemented in place by the cement or concrete plug at the end of the shoe. Additionally, U.S. Pat. No. 1,994,846, to Baker, also discloses a float shoe having a plug closure at the end which closes the opening through the shoe while the casing is being floated into place. The plug may thereafter be forced out to permit the cementing of the well. Additionally, U.S. Pat. No. 4,442,894, to Callihan et al, discloses a cementing float valve and wiping plug combination wherein the valve structure is indirectly secured to the valve housing, which is metallic, by means of cement.
Subsequent to a cementing operation, it often is desirable to drill through the cement shoe or collar. For such purpose, a drill bit is run in on a drill string within the casing and rotated at high rpm to drill through such valve structures within the float equipment. Accordingly, it has been found that valve head and seat arrangements and other components of the valving structures in float equipment which are fabricated using metallic components are highly disadvantageous to speedy and economical drilling operations for drilling up such valve collars and shoes, and, in combination with the cementitious material which is utilized in such devices, makes the drill up operation extremely time consuming, in many instances.
While there are recognized advantages in using cementitious material to secure the valve structure to the interior of the metallic housing for such float shoes and collars, the space afforded for such cementitious material greatly reduces the fluid flow area within the valve structure. Additionally, even the best and highest quality cementitious material has a tendency to be comparatively highly porous, thus providing which is sometimes referred to in the industry as a "microannulus" area through the cementitious material which prevents the valving structure from affording a positive and complete seal to completely eliminate fluid flow through and across the device.
In prior art float valve structures used in collars and shoes there is provided a biasing means to urge the valve head to its companion valve seat in the closed position, such biasing means typically being a compressible spring, or the like. Such spring elements will be of a metallic substance which are housed within such prior art valving structures such that they are generally always exposed to and in contact with the corrosive fluids in the well. Additionally, the high velocity, cementitious and other fluids which are pumped through the valving structure have, in prior art devices, come into contact with such biasing spring members to subject them to errosion, thus greatly reducing their life expectancy and performance integrity.
Still another problem encountered in prior art designs of such valving structures in float equipment for subterranean wells is the inability of such devices to fully shift the valve head away from the valve seat when cementitious fluids are pumped through the casing string at reduced flow rates, on the order of about 2 to 3 barrels per minute, thus resulting in the valve head of the device being within the flow path of the fluid such that the turbulence of the fluid causes valve throttling, resulting in abrasion and errosion of the head structure and a reduction of the fluid sealing integrity between the head and seat arrangement.
The present invention provides a device which overcomes the above-described problems encountered in prior art structures.